Summary

Fixes two xImage background-task reliability issues found during public-beta QA:

1. Nested xImage paths break image embedding. Object keys encode nested paths with extra slash segments, for example project/db/items/doc1/meta/image/image.png, but embed_image_task() unpacked exactly six slash-separated values.
2. If retrieve_from_minio() fails, the exception handler writes public_url before it has been assigned, so the task raises UnboundLocalError instead of marking the field as failed.

Changes

• Add parse_image_object_key() to preserve nested object paths as dot-notated EmbJSON paths.
• Generate public_url before the MinIO retrieval try block so failure handling can still write a useful failed status payload.

Validation

Nested object-key parser check:

proj/db/items/doc1/image/image.png => ('proj', 'db', 'items', 'doc1', 'image')
proj/db/items/doc1/meta/image/image.png => ('proj', 'db', 'items', 'doc1', 'meta.image')
proj/db/items/doc1/items/0/image/image.png => ('proj', 'db', 'items', 'doc1', 'items.0.image')
PASS

Failure-handler check with MinIO retrieval stubbed to fail:

[(({'_id': 'doc1'}, {'$set': {'image.xImage.status': 'failed', 'image.xImage.data': 'http://minio/proj/db/items/doc1/image/image.png'}}), {})]
PASS

Also ran:

python3 -m compileall -q celery_tasks/image_tasks.py